fix(proxy): return display model when request is auto

When request model is 'auto', use config.defaultModel for response
model field instead of echoing 'auto'. Covers CURSOR_BRIDGE_DEFAULT_MODEL
set case; when unset, response remains 'auto'.

- chat-completions.ts: displayModel for streaming and non-streaming
- anthropic-messages.ts: same logic for message_start and final json
- server.test: add tests for display model and echo-auto cases